Riser flow control

ABSTRACT

A deep sea mining method includes: excavating matter at a bottom of a body of water; operating a riser system for transporting a slurry of matter and water; transporting the slurry from the bottom of a body of water upwards to a slurry processing base; and maintaining a controlled riser system pressure higher than the environmental pressure inside the riser system for avoiding forming of gas and expanding of gas contained in the slurry during transporting the slurry.

BACKGROUND

The present invention relates to a deep sea mining method comprising;

-   -   excavating matter at a bottom of a body of water,     -   operating a riser system for transporting a slurry of matter and         water,     -   transporting the slurry from the bottom of a body of water         upwards to a slurry processing base.

EP2570340 generally relates to a device for capturing fluids lighter than water escaping from an underwater source. The device is useful for collecting and temporarily storing oil and/or gas escaping from an out-of-control deep-sea borehole. EP2570340 intends to provide an improved device for collecting and temporarily storing fluids rising from an underwater source, in particular with respect to the device's ability to handle fluids containing gas. EP2570340 discloses a riser tube having flow restrictors, such as, e.g. choke disks, arranged in its interior for restricting the flow of the fluids. According to EP2570340, gas-containing fluids may be transferred from the seabed to the underwater buffer reservoir, without requiring separation of the gas fraction at the seabed. This simplifies the structure to be deployed over the underwater source. The flow restrictors, which are preferably arranged at regular intervals along the length of the riser tube, reduce the velocity of the fluid.

U.S. Pat. No. 6,412,562 relates to electrical submersible well pumps inside a riser. As the production fluids flow up the well, the pressure drops and gases that were in solution become free gases. This invention of U.S. Pat. No. 6,412,562 is able to artificially boost the riser pressure to increase production and force some of the free gases back into solution.

WO 2010/092145 A1 relates to a method for converting hydrates buried in a water bottom into a marketable hydrocarbon composition. WO 2010/092145 A1 is not concerned with pressure control in a riser, let alone pressure control along the length of a riser. Instead WO 2010/092145 A1 seeks to reduce the amount of power required to lift the slurry and provides therefore a slurry lifting assembly comprising a slurry pump, which is actuated by the tailings stream.

WO2013050138 (A2) relates to a riser system for transporting the slurry from a position adjacent to the seabed to a position adjacent to the sea surface. WO2013050138 (A2) aims to provide a riser system which can operate effectively and provides therefore a riser system for transporting the slurry from a position adjacent to the seabed to a position adjacent to the sea surface, the riser system comprising first and second risers; a slurry pump system to transport slurry up one of the risers and a waste water pump system to return waste water down one of the risers; wherein the slurry pump system and the waste water pump system are selectively connectable to each of the risers to allow each riser to be either a slurry riser or a waste water riser. With this arrangement, if a slurry riser develops a leak partway along its length the waste water riser can be converted into a slurry riser such that operation can continue. WO2013050138 (A2) has for a drawback that gas hydrates entering the system will start dissociating under the influence of the reducing pressure during the assent to the surface. This impedes control of the flow in a riser.

SUMMARY OF THE INVENTION

When sediment or minerals are excavated at the bottom of a sea, which is considered a high pressure environment, the minerals are fluidized in a slurry and pumped via a riser from the seafloor to a vessel, which is considered a low pressure environment. If the pumped mixture is compressible, this mixture will expand when pressure lowers. Some types of sediments contain enclosed gas. While the absolute pressure in the riser system drops during transport for the seafloor to the vessel, these enclosed gasses might become free gasses. Free gas can impose a problem for the functioning of certain devices in the riser system, such as centrifugal pumps, but can also lead to reduced controllability of the slurry flow, as the slurry velocity rises and fluctuates with the expansion of gasses.

The invention aims to provide a deep sea mining method having an improved controllability of the slurry flow.

Another object of the invention is to improve a known deep sea mining method in that a problem associated therewith is at least partly solved.

Yet another object of the invention is to provide an alternative deep sea mining method.

According to a first aspect of the invention this is realized with a deep sea mining method comprising;

-   -   excavating matter at a bottom of a body of water,     -   operating a riser system for transporting a slurry of matter and         water,     -   transporting the slurry from the bottom of a body of water         upwards to a slurry processing base,     -   maintaining a controlled riser system pressure, higher than the         environmental pressure, inside the riser system for avoiding         forming or release of gas and expanding of gas contained in the         slurry during transporting the slurry.

Maintaining a higher pressure inside the riser system beneficially adds stability o the slurry flow and reduces creation of free gas from sediments.

Where the invention relates to mining operations and a slurry flow, EP2570340 and U.S. Pat. No. 6,412,562 distinctively relate to operations concerning an oil- or gas-flow. The oil- and gas operations is a different technical field compared with mining operations mainly because of the different flow which imposes different requirements to systems and operations. The mining slurry flow differs with a oil or gas flow at least in that the slurry has a ranging density and is inhomogeneous. In addition, the solid content of the mining slurry is much higher. From system and operation perspective, the oil and gas operations do not require active pumping since the body of water provides an overpressure. Also means in relation to risk of a blow out in a oil and gas system is not relevant for a mining system.

Deep sea here means seas having a depth of at least 500 meters, preferably at least 1000 meters.

In an embodiment, the method comprises the step measuring the pressure for providing a riser system pressure signal, and controlling the pressure in response to said pressure signal. Preferably, the method comprises controlling the riser system pressure to a predefined riser system pressure threshold. Such a threshold may be dependent on the vertical position in the riser system.

In an embodiment, the maintaining a controlled riser system pressure comprises providing pressure control means. The pressure control means ma comprise active and/or passive means.

In an embodiment of the method, the pressure control means are selected from restriction, systems of bends, turbines, pumps used as turbines, and pumps.

In an embodiment of the method, a series of pressure control means is provided. This enables to control pressure along the length of the riser system.

In an embodiment of the method, the series of pressure control means is arranged along the length of the riser system for controlling pressure in the riser system. This improves pressure control along the length of the riser system.

In an embodiment of the method, adjacent control means are arranged at a predefined mutual distance. This even more improves pressure control along the length of the riser system.

In an embodiment, the method comprises recovering energy from the transported slurry through the pressure control means. This enables to recover possible energy surplus downstream, which surplus has been spend to pressure control upstream.

The matter may comprise gashydrates.

According to a further aspect of the invention this is realized with a deep sea mining system for use in the method according to the invention, the system comprising;

-   -   an excavating system for excavating matter at a bottom of the         body of water,     -   a riser system for transporting a slurry of matter and water         from the bottom of a body of water upwards to a slurry         processing base, the riser system comprising an adjustable pump         means proximate a bottom of a body of water,     -   pressure control means for maintaining a controlled riser system         pressure higher than the environmental pressure inside the riser         system for avoiding forming or release of gas and expanding of         gas contained in the slurry during transporting the slurry.

In an embodiment, the deep sea mining system comprises a series of pressure control means arranged along the length of the riser pipe for controlling pressure in the riser system. This improves pressure control along the length of the riser system.

In an embodiment of the deep sea mining system, adjacent control means are arranged at a predefined mutual distance.

In an embodiment of the deep sea mining system, the pressure control means comprise active and/or passive means.

In an embodiment of the deep sea mining system, the pressure control means are selected from restrictor, systems of bends, turbines, pumps used as turbines, and pumps.

In an embodiment, the deep sea mining system comprises;

-   -   measuring means for measuring the riser system pressure in order         to provide a riser system pressure signal,     -   a control unit operationally coupled with the measuring means         and the pressure control means for controlling the riser system         pressure in response to said pressure signal.

The invention further relates to a device comprising one or more of the characterising features described in the description and/or shown in the attached drawings.

The invention further relates to a method comprising one or more of the characterising features described in the description and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order to provide additional advantages.

DESCRIPTION OF THE DRAWINGS

The invention will be further duel dated referring to an preferred embodiment shown in the schematic drawings wherein shown in:

FIG. 1 in side view a deep sea mining system according to the invention,

FIG. 2 in side view, a further embodiment a deep sea mining system according to the invention; and

FIG. 3 a detail of a the mining system of FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a deep sea mining system 1 for use in a deep sea mining method. The mining system 1 comprises a excavating system 7 for excavating matter 2 at a bottom 9 of a body of water 3. The excavating system 7 has a boom member provided with a cutting member at its lower end. The excavating system 7, may float or be supported by the bottom 9 in a rolling manner.

The mining system 1 comprises a riser system 4 for transporting a slurry 13 of matter and water from the bottom 9 Of a body of water upwards to a slurry processing base 6. The riser system 4 comprises an adjustable pump means 11 proximate the bottom 9 of a body of water for pressurized transport of the slurry 13. The base 6 is for example a floating vessel (FIG. 1) or a floating platform (FIG. 2) or a stationary platform (not shown).

The mining system 1 comprises pressure control means 5 a, 5 b, 5 c, 5 d for maintaining a controlled riser system pressure higher than the environmental pressure inside the riser system for avoiding forming of gas and expanding of gas contained in the slurry during transporting the slurry. In this case, the riser system 4 is provided with a series of pressure control means 5 a, 5 b, 5 c, 5 d arranged along the length of the riser system 4 for controlling pressure in the riser system. Here, adjacent control means 5 a, 5 b, 5 c, 5 d are arranged at a predefined mutual distance d. The riser system 4 is a pipe or line Which extends from the bottom 9 of the body of water 3 up to the slurry processing base 6 at the water surface 10.

The pressure control means 5 a, 5 b, 5 c, 5 d may be active and/or passive means which take the form of a (not shown) restrictor, systems of bends, turbines, pumps used as turbines, and pumps 12 a, 12 b schematically shown in FIG. 3.

The deep sea mining system 1 may comprise measuring means (not shown) for measuring the riser system pressure in order to provide a riser system pressure signal, as well as a control unit (not shown) operationally coupled with the measuring means and the pressure control means for controlling the riser system pressure in response to said pressure signal.

It will also be obvious after the above description and drawings are included to illustrate some embodiments of the invention, and not to limit the scope of protection. Starting from this disclosure, many more embodiments will be evident to a skilled person which are within the scope of protection and the essence of this invention and which are obvious combinations of prior art techniques and the disclosure of this patent. 

1-17. (canceled)
 18. Deep sea mining method comprising; excavating matter (2) at a bottom (9) of a body of water (3), operating a riser system (4) for transporting a slurry (13) of matter and water, transporting the slurry from the bottom of a body of water upwards to a slurry processing base (6), maintaining a controlled riser system pressure, higher than the environmental pressure, inside the riser system for avoiding forming or release of gas and expanding of gas contained in the slurry during transporting the slurry.
 19. Method according to claim 18, comprising the step measuring the pressure for providing a riser system pressure signal, and controlling the pressure in response to said pressure signal.
 20. Method according to claim 19, comprising controlling the riser system pressure to a predefined riser system pressure threshold.
 21. Method according to claim 18, wherein the maintaining a controlled riser system pressure comprises providing pressure control means (5 a, 5 b, 5 c, 5 d, 11).
 22. Method according to claim 21, wherein the pressure control means comprise active and/or passive means.
 23. Method according to claim 21, wherein the pressure control means are selected from restriction, systems of bends, turbines, pumps used as turbines, and pumps.
 24. Method according to claim 21, wherein a series of pressure control means is provided.
 25. Method according to claim 24, wherein the series of pressure control means is arranged along the length of the riser system for controlling pressure in the riser system.
 26. Method according to claim 24, wherein adjacent control means are arranged at a predefined mutual distance (d).
 27. Method according to claim 21, comprising recovering energy from the transported slurry through the pressure control means.
 28. Method according to claim 18, wherein the matter comprises gashydrates.
 29. Deep sea mining system (1) for use in a method according to claim 18, the system comprising; an excavating system (7) for excavating matter (2) at a bottom (9) of a body of water (3), a riser system (4) for transporting a slurry (13) of matter and water from the bottom of a body of water upwards to a slurry processing base (6), the riser system comprising an adjustable pump means (11) proximate the bottom of a body of water, pressure control means (5 a, 5 b, 5 c, 5 d) for maintaining a controlled riser system pressure higher than the environmental pressure inside the riser system for avoiding forming or release of gas and expanding of gas contained in the slurry during transporting the slurry.
 30. Deep sea mining system according to claim 29, comprising a series of pressure control means arranged along the length of the riser pipe for controlling pressure in the riser pipe.
 31. Deep sea mining system according claim 30, wherein adjacent control means are arranged at a predefined mutual distance (d).
 32. Deep sea mining system according to claim 29, wherein the pressure control means comprise active and/or passive means.
 33. Deep sea mining system according to claim 29, wherein the pressure control means are selected from restrictor, systems of bends, turbines, pumps used as turbines, and pumps.
 34. Deep sea mining system according to claim 29, comprising; measuring means for measuring the riser system pressure in order to provide a riser system pressure signal, a control unit operationally coupled with the measuring means and the pressure control means for controlling the riser system pressure in response to said pressure signal.
 35. Deep sea mining system (1) for use in a method according to claim 19, the system comprising; an excavating system (7) for excavating matter (2) at a bottom (9) of a body of water (3), a riser system (4) for transporting a slurry (13) of matter and water from the bottom of a body of water upwards to a slurry processing base (6), the riser system comprising an adjustable pump means (11) proximate the bottom of a body of water, pressure control means (5 a, 5 b, 5 c, 5 d) for maintaining a controlled riser system pressure higher than the environmental pressure inside the riser system for avoiding forming or release of gas and expanding of gas contained in the slurry during transporting the slurry.
 36. Deep sea mining system (1) for use in a method according to claim 20, the system comprising; an excavating system (7) for excavating matter (2) at a bottom (9) of a body of water (3), a riser system (4) for transporting a slurry (13) of matter and water from the bottom of a body of water upwards to a slurry processing base (6), the riser system comprising an adjustable pump means (11) proximate the bottom of a body of water, pressure control means (5 a, 5 b, 5 c, 5 d) for maintaining a controlled riser system pressure higher than the environmental pressure inside the riser system for avoiding forming or release of gas and expanding of gas contained in the slurry during transporting the slurry.
 37. Deep sea mining system (1) for use in a method according to claim 21, the system comprising; an excavating system (7) for excavating matter (2) at a bottom (9) of a body of water (3), a riser system (4) for transporting a slurry (13) of matter and water from the bottom of a body of water upwards to a slurry processing base (6), the riser system comprising an adjustable pump means (11) proximate the bottom of a body of water, pressure control means (5 a, 5 b, 5 c, 5 d) for maintaining a controlled riser system pressure higher than the environmental pressure inside the riser system for avoiding forming or release of gas and expanding of gas contained in the slurry during transporting the slurry. 